Polyimide (PI) is a heat resistive material which is widely used in industrial applications. However industrial application is sometimes limited by undesirable properties of the PI surface, for example, poor adhesion and electrical conductivity. These properties need to be improved by the modification of surfaces. Surfaces of PI films have been irradiated with high energy ion beams to change their surface conductivities and adhesion. The ion beam is extracted from the ECRIS (Electron Cyclotron Resonance Ion Source) with circular hole electrodes. The ECR plasma source is equipped with 2.45-GHz magnetron rf source with 1 kW maximum power output, and two solenoid magnets. Typical working pressure of argon gas is ~ 5 ×10E-5 torr. In this paper, we present the experimental result on the relation between Ar ion does and the surface resistivity of the PI film. The ion dose on the PI film was calculated with ion beam profile measured by a Faraday cup. Uniform ion dose areas were found obtaining ion beam profiles in various changing extraction voltages and micro-wave powers. PI film was modified to improve the surface electrical conductivity at an energy range of 5-20 kV. The surface resistivity at 15 kV with a dose of 1 × 10E16 ions/cm2 was reduced by 8 orders of magnitude and maintained for more than 3 months. The contact angle of water drops on the Ar ion implanted PI film decreases from 60o to 40o as the surface resistivity decreases.